Although hepatitis C virus (HCV) is a leading cause of morbidity and mortality worldwide, the effects of viral gene expression on infected cells remain unclear in vivo. Previously, we reported the construction of transgenic mice expressing HCV structural proteins (core, E1 and E2) and showed that expression of HCV structural proteins is not directly cytopathic in this animal model. Using DNA immunization, we were able to induce antibody and T cell proliferative responses but not cytotoxic T cell response against HCV structural proteins in these transgenic animals, suggesting that tolerance at the T cell level is more difficult to break. Our laboratory is also generating transgenic mice expressing HCV full-length polyprotein and has developed a system for inducible expression of HCV transgene using the tetracycline-inducible system. Using this method, transgenes could be expressed in a time- and tissue-specific manner and it is also possible to avoid the thymic selection process that occurs during embryonic development. This is of great advantage for studying the immune responses against foreign antigens such as viral proteins since transgenic mice expressing viral genes are usually found to be tolerant to the viral gene products. We have developed two transgenic mouse models, each conditionally expressing SV-40 T antigen (Tag) or LacZ . To obtain these animals, we first generated transgenic mice expressing the tTA transactivator under the control of liver-specific albumin promoter (AlbtTA) or mouse urinary protein promoter (MuptTA) and then cross these animals with mice containing tetTag or tetLacZ transgene. Offsprings were genotyped and subjected to tetracycline (Tc) treatment by subcutaneous implantation of slow release Tc pellets. Expression of the transgene was analyzed 21 days after Tc implantation. Tag expression was detected in the liver of animals that were double-positive for either AlbtTA/tetTag or MuptTA/tetTag. In both cases transgene expression was confined to the liver only and could be completely abolished by Tc treatment as evidenced by absence of transgene transcription by RT-PCR. Tag protein synthesis resumed upon withdrawal of Tc. Similarly, expression of LacZ in mice that were double-positive for MuptTA/tetLacZ was tightly regulated by Tc treatment. We are currently evaluating the regulated expression of HCV proteins in the liver using this model system. These animals will provide a useful animal model not only to address issues of immunopathogenesis and cytopathic potential of HCV gene products but also to study HCV replication in vivo. - Cytopathic Effect/Inducible Expression/Immune Response/Animal Model